1. Field of the Invention
The invention is situated in the field of materials handling technology and it concerns a method and arrangement for producing cross stacks made up of serially supplied, flat objects, in particular of printed products, such as, for example, newspapers or periodicals.
2. Description of Related Art Group
In a stack of flat objects all having approximately the same shape, the objects are lying substantially parallel to one another, directly adjacent to one another and they are aligned with one another (same edges of all objects are parallel to one another), such that the stack has the same base area as each individual object. Stability and handling of such stacks is very much dependent on the uniformity of the thickness of the objects over their flat expanse. Objects that have a regular thickness can be stacked in a more stable manner than objects comprising thicker and thinner areas. Stack instabilities resulting from irregular object thicknesses can be avoided for specific object forms by aligning the objects within the stack not in the most restrictive sense, but in such a manner that edge zones of differing thickness are positioned on top of one another, so that the stack being produced obtains an as equal as possible height on all sides and the objects in it are aligned as parallel with one another as possible.
An example of flat objects with non-uniform thickness, which can be stacked in stable stacks by the method mentioned above, are folded printed products with a rectangular or square shape. Products of this type are usually stacked in so-called cross stacks, i.e., on a first stacked group of products having an equal orientation (same edges are superimposed) a second group of products again having an equal orientation is positioned such that the same edges of the products of the first and second groups are located opposite one another and that the thickest product corners in the first and second group are lying diagonally opposite one another. On to the second group a third group of products having the same orientation as the products of the first group is stacked, and so on. The products of neighbouring groups are therefore rotated relative to one another by 180xc2x0 around an axis perpendicular to the product surfaces (stacking axis).
For stacking, printed products are, for example, transported in an imbricated formation loosely lying on a conveyor belt, in which the leading product edges are positioned on top, towards a stacking shaft, which is open on top, and then one product after the other is pushed over the stacking shaft opening. Depending on the design of the shaft, the products are pushed directly onto a stacking table, onto a stack being produced, or drop from the stacking shaft opening onto the stacking table or stack. It is also known to convey the printed products again in an imbricated formation, but individually held on their leading edges, toward the stacking shaft, to pull them over the shaft opening, and to then release them.
For producing cross stacks, usually the stacking shaft or stacking table is rotated by 180xc2x0 around a vertical rotation axis (stacking axis) after deposition of each group of products (stack section or layer). During rotation of the stacking shaft, the products continuing to be supplied are usually stacked on an auxiliary table. The auxiliary table is lowered after the rotation and is then laterally removed from the stack. All the same, product supply has to be briefly interrupted between each two individual groups (for re-positioning the auxiliary table). For rotating the stacking shaft, for interrupting the product supply, and for positioning of the auxiliary table many moving parts are necessary, which renders the corresponding devices complicated and increases maintenance. Examples of such stacking methods are described in the publications CH-539569, DE-2752513 (or GB-1568752) or EP-0586802 (or U.S. Pat. No. 5,370,382).
According to the method described in the publication EP-0854105 (or U.S. Pat. No. 6,139,252), a first stream of individually held printed products, which are all oriented in the same way, is transformed into a second stream of individually held printed products, wherein the printed products in the second stream are rotated by 180xc2x0 and held on opposite edges in alternating groups, i.e. the products of the second stream are arranged in the same way as in a cross stack. The stream transformation is implemented by transferring the products of every second group to grippers of an auxiliary conveying system, by rotating the transferred products by conveying them along the twisted conveying path of the auxiliary conveying system, and by transferring the rotated products to the original conveying system gripping the products on an edge situated opposite the originally held edge. The products of the other groups are not transferred to the auxiliary conveying system and are, therefore, not rotated. Producing cross stacks from products supplied in a such transformed product stream is obviously significantly simpler than producing cross stacks from products being supplied having all the same orientation. However, stream transformation as described above sets high demands with respect to the equipment required and with respect to the alignment and synchronization of the conveying systems co-operating for the product transfer.
An object of the invention is to create a method and an arrangement for producing cross stacks from flat objects, in particular from printed products, being supplied serially and individually held gripped and all having the same orientation. The method and the arrangement are to be completely independent of whether the cross stack sections or stack layers (groups of objects) are large or small (if so required only comprising one product), and also independent of whether these stack sections have a uniform or a varying size. The method is to be simple and it has to make short cycle times possible. The arrangement is to be simple and to comprise as few moving parts as possible. The method and arrangement are to function without the necessity of temporarily taking over the objects by further grippers of an auxiliary conveying system such that the above mentioned alignment and synchronization difficulties are prevented.
According to the invention, the flat objects have a shape suitable for cross stacking (for example, folded, rectangular or square printed products) and are supplied for stacking individually held, behind each other and all having the same orientation. The flat objects are subjected in alternating groups to a first step sequence or a second step sequence prior to being positioned on a stack being produced. The two step sequences differ from one another such that a cross stack is produced when the alternating groups are positioned in a stacking device. This means that, prior to being positioned in a stacking device, the objects supplied serially and all oriented the same are handled differently in alternating groups in such a different manner, that after such handling they can be stacked in a cross stack without any further measures. The cross stack comprises the alternating groups of objects as stack sections, wherein in two neighbouring stack sections the objects are rotated by 180xc2x0 around the stacking axis (perpendicular to the object surfaces).
Of the two step sequences mentioned:
both comprise a release step, in which the objects are released from being held gripped, wherein for at least one of the step sequences the release step comprises transferring the objects from held conveyance to lying conveyance in an imbricated formation (i.e. imbricated stream on a conveying surface or between two co-operating conveying surfaces) and wherein for the other step sequence the release step may comprise positioning the objects directly into a stacking device; and,
together the two comprise rotation steps, which differ such that between objects having undergone the first step sequence and objects having undergone the second step sequence there is a rotation difference of 180xc2x0 around an axis perpendicular to the object surfaces.
The rotation steps producing the required rotation difference are to be carried out correspondingly differently in the two step sequences and they may comprise:
rotating the objects during held conveyance;
transferring the objects from held conveyance to lying conveyance and rotating them simultaneously; and,
rotating the objects during the lying conveyance.
A rotation difference of 180xc2x0 around an axis perpendicular to the object surfaces can be implemented by a rotation step in one of the step sequences only. It can also be implemented as two partial rotations, which are carried out in opposite directions in both of the two step sequences. The rotation difference may also be implemented as combined rotations each by 180xc2x0 around two axes parallel to the object surfaces and perpendicular to one another, wherein once again each individual one of the two rotations can be carried out as a complete rotation in one step sequence or as two partial rotations in the two step sequences.
The stacking device advantageously used for the method according to the invention is a stacking shaft to which, if so required, products can be supplied from two opposite directions and/or on two stacking levels located one above the other.